This invention relates to a method and apparatus for processing component signals to preserve high frequency intensity information.
FIG. 1 shows a conventional color video camera and a conventional color cathode ray tube (CRT) display device 6. Camera 2 includes three linear sensors 4R, 4G and 4B, which generate respective color component signals R', G' and B'. The voltages of these three signals are proportional to the intensity of red, green and blue light respectively in the camera's focal plane. However, the intensity of light emitted by the screen of a conventional CRT is not linearly related to the voltage of the video signal that is applied to the electron gun of the CRT. In the case of a color CRT, the intensity of light emitted by the CRT is given by ##EQU1## where R, G and B are the driving voltages applied to the red, green and blue electron guns respectively, ** is the exponentiation operator and GAMMA is a constant (2.2 in the case of the NTSC system).
Because of this relationship between electron gun driving voltage and emitted light intensity, the video camera shown in FIG. 1 incorporates GAMMA correction circuits 5R, 5G and 5B, so that the red component signal R outputted by the camera is proportional to R'**(1/GAMMA), and similarly for G and B. The R, G and B color component signals provided by the camera may be used, with suitable amplification, to drive the CRT directly, as shown in dashed lines, and the intensity of red, green and blue light emitted by the CRT would be proportional to R', G' and B' respectively. However, most color television standards, such as NTSC, PAL and RP125, encode visual information as luminance (Y) and chrominance, or chroma (R-Y and B-Y), where EQU Y=.multidot.299*R+.multidot.587*G+.multidot.114*B
Therefore, video camera 2 includes a resistive encoding matrix 8 that converts the R, G and B component signals to luminance and chroma component signals and the display device 6 includes a decoding matrix 9 that receives the Y, R-Y and B-Y signals and reconstructs the R, G and B component signals therefrom.
If R, G and B each range in value from 0 to 1, and R, G and B are each equal to 1, so that white light is emitted, Y is equal to 1 and the emitted light intensity I is equal to 1. However, because the emitted light intensity is a non-linear function of R, G and B, the luminance component, Y, is not sufficient to describe the intensity of the light emitted by the CRT. Thus, a given Y value will result in a higher intensity when combined with large chroma values (large absolute values for R-Y and/or B-Y) than when combined with small chroma values. For example, a saturated full brightness red (R=1, G=0 and B=0) has a Y value of .299 and provides an intensity value of .299, whereas a gray for which R=.299, G=.299 and B=.299 also provides a Y value of .299 but results in an intensity of .299**GAMMA, or .070 for GAMMA=2.2.
A problem with the non-linearity of the relationship between emitted intensity and R, G and B arises when the chroma component signals are filtered to a lower bandwidth than the luminance component signal. If color component signals R, G and B are used to drive a high resolution CRT display, and the value of R within a selected area of the field is 1 and elsewhere it is 0 and the values of G and B are 0 throughout the field, so that the CRT displays an area of saturated red against a black background, the peak value of Y is .299 and the peak value of I is .299. If these color component signals are converted to the NTSC standard, in which the Y component signal has a potential bandwidth of 4.2 MHz and the chroma component signals are limited to a bandwidth of 1.2 MHz, and the area of the field that is red is a vertical line that is at least as wide as allowed by the bandwidth of the chroma channels, the values of Y and I within the area of the red line are the same as in the case of the high resolution display The peak values of R-Y and B-Y are .701 and -.299 respectively. If, however, the line was as narrow as allowed by the luminance channel bandwidth, the chroma filters would spread out the chroma signals by a factor of three or so, and reduce the peak values of the chroma signals by the same factor. Accordingly, while the peak value of Y is still .299, the peak values of R-Y and B-Y are .234 and -100 respectively, and the peak intensity is .095 for GAMMA equal to 2.2., or only about one-third of the intensity value for the wider line. This problem of reduced intensity is not limited to the case in which the signals are filtered in the horizontal direction, and arises also with vertical filtering, for example when component signals for driving a high resolution display, which may have more than a thousand lines per frame, are converted to a broadcast television standard having only about 500 or 600 lines per frame. In the case of the PAL system, in which GAMMA is equal to 2.8, the problem is even more severe.